With one of your flyer pages on view, click the Format tab in the Menubar and then Background in the dropdown menu.
You can select a default offering or use the More colors link to see a wider choice to choose from.
Select a colour then click OK.
This then shows a range of colours/gradients etc relating to your colour selection.
You can click a colour/gradient to select it for the showing page, or use the small arrow in a showing colour/gradient to either apply it to the page showing or to all.
Should you just apply it to the one showing, opening other pages allows you to go through the process to select a colour per page.
Click the image below to see a short video of the process.
My example uses Publisher 2003.